US Ambassador to the UN Nikki Haley said the US blamed Russia for the attempted killing of a former Russian military-intelligence agent in the UK earlier this month using a deadly nerve agent.

“The United States believes that Russia is responsible for the attack on two people in the United Kingdom using a military-grade nerve agent,” Haley said before the UN Security Council on Wednesday, according to Voice of America national-security correspondent Jeff Seldin. “The United States stands in absolutely solidarity with Great Britain.”

The former agent, Sergei Skripal, and his daughter were found collapsed on a bench in southern England on March 4. They were poisoned with Novichok, a Soviet-era nerve agent, that was smeared on his car door, British government sources told The Daily Mail.

“Alone, Russia’s crime is worthy of this council’s action,” Haley said. “But this is not an isolated incident. The assassination attempt in Salisbury is part of an alarming increase in the use of chemical weapons.”

A Russian chemist who helped develop the agent said he had no doubt that Russian President Vladimir Putin was behind the poisoning, as Moscow has tight control over its Novichok stockpile and because the agent is too complicated to be weaponised by a nonstate actor.

“The Kremlin all the time, like all criminals, denying – it doesn’t mean anything,” the chemist, Vil Mirzayanov, told Reuters. Mirzayanov now lives in Princeton, New Jersey, having been in exile for more than 20 years.

The UK has already responded to the incident. On Wednesday, Prime Minister Theresa May announced the expulsion of 23 Russian diplomats, who she said were Russian spies. May also said the UK would freeze the assets of Russians living in the UK under suspicion of undermining British interests.

“For those who wish to do us harm, my message is clear. You are not welcome here,” May said. “This will be the single biggest expulsion for over 30 years and it reflects the fact that this is not the first time that the Russian State has acted against our country.”

May also said all high-level contact between the UK and Russian governments would end.

German Chancellor Angela Merkel and Trump have said their governments were taking the British government’s assessment of the attack extremely seriously. Others have been more cautious about assigning blame for the poisoning.

A spokesperson for British opposition leader Jeremy Corbyn said on Wednesday he did not believe there was sufficient proof yet to confirm Russia was behind the attack. The French government also said Wednesday that it wanted firm proof of Russia’s involvement in the poisoning before it acted in support of the British government.

“We don’t do fantasy politics. Once the elements are proven, then the time will come for decisions to be made,” French government spokesman Benjamin Griveaux told reporters shortly after May’s announcement.

Russia swiftly rebuked May over her government’s actions. Russia’s ambassador to the UK, Alexander Yakovenko, said the UK’s actions were “unacceptable” and “a very serious provocation.”